Power system and shunt for reducing harmonics therein



1941. H. w. WAHLQUISi' 2,241,831

POWER SYSTEI AND SHUNT FOR REDUCING HARIOIIICS THBRBI Filed larch 20,1940 A 3 Shasta-Shoot 1 INVENTOR. fil'yal'iffi a/ilyar BY w lt due? a MATTORNEY.

' May 13, 1941. H. w. WAHLQUIST 2,241,331

POWER SYSTBI AND SHUNT FOR REDUCING HARIONICS THEREIN I'ilod larch 20,1940 3 Shasta-Shoot 2 zaz INVENTOR. fiuya WlVafi/yzzzlrz BY 92 da&

M ATTORNEY.

May 13, 1941. H. w. WAHLQUIST 2,241,831

Pom SYS'IBI AND SHUNT FOR REDUCING HARIONICS THERB IN Filod larch 20.1940 s shuts-shut 3 FIIQIIENCY' (7C! [-5 PER CWO owns If .m

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M ATTORNEY.

Patented May 13, 1941 UNITED STATES-PATENT OFFICE POWER SYSTEM AND SHUNTFOR REDUCING HABMONICS THEREIN Hugo W. Wahlquist, Mount Vernon, N. Y.Application March '20, 1949, Serial No. 334,940 Claims. (Cl. 171-97)This invention relates to power systems and shunts for reducingharmonics therein.- Although not restricted thereto, the invention isparticularly applicable to power lines having considerable distributedcapacitance fed bysupply systems having considerable inductance, suchlines having, by virtue of such capacitance and inductance, a normalresonance at a frequency in the voice frequency range and especiallybetween 500-and 2,000'cycles. Further,-.while .the most frequent reasonfor using such shunts is to prevent interferencewith parallelingtelephone or other communication lines, such shunts are often desirableto improve the power transmission efllciency of power lines. Largeamounts of harmonics may create serious fluctuations in the voltage,noise in radio sets connected to the line,-

increased losses, and other objectionable effects.

This application is a continuation-in-part of 20 For many power systems,whether of the above Patent No. 2,212,963, issued August 27, 1940.

specific type or not, it is advantageous to use a shunt comprising aninductance element in series with a capacitor. However, as the impedanceof the ordinary inductance element increases proportionately to thefrequency, it follows that such a eries shunt offers high reactance tothe flow of high frequency current. Consequently, while the shunt mayprevent resonance between the line and the supply system at somefrequency over 500 and may act as a short circuit for currents'of theparticular frequency at which the shunt itself is resonant, it may notto any great extent by-pass other high-frequency current originating ineither the supply system or the loads thereon.

One of the object of the invention is to provide an improved form ofshunt or network of shunts which destroy resonance in both the balancedand residual circuits at any frequency, whereby the necessity forelaborate and careful investigation of the line characteristics isavoided.

Further, line characteristics change from time to time as the result ofline extensions and connected load. Such changes also change theresonant frequencies so that a device is required which will destroyresonance irrespective of frequency.

Another general object of the invention is to provide an inductiveelement, the inductance of which does not increase proportionately withfrequency. Such an inductive element reduce the reactance of the shuntfor frequencies above that at which theshunt itself is resonant, andmakes the shunt act more nearly like a resistance above the resonantfrequency.

Such a variable inductance may be constructed by using a high-loss ironcore or by placing a resistance in parallel with an air-core coil, or

' by a combination of both methods, i. e., an ironcore coil with aresistance in parallel therewith, or by short-circuited turns using anair core, or by short-circuiting the turns through a resistance, if aniron core is used. Shunts of the above typemay be used to advantage withrecliners and other loads creating large amounts of harmonies. Usuallythe volume of harmonics created by such load is so large that a singlesimple capacitor shunt cannot reduce them suiiiciently to give goodtelephone service in nearby lines. The above-mentioned variableinductance shunts are, however, so much more efflcient that they can beused in many cases where otherwise a plurality of tuned shunts wouldhave to be employed. When used to reduce harmonics created by oneparticular load, such as a large rectifier, it is desirable to place theshunt adjacent the load. However, the harmonics generated by the usualloads, such as those arising from transformer exciting currents fromtransformers distributed along the line, can be taken care of by myimproved shunt substantially irrespective of the location of the shunt,where resonance occurs in the voice frequency range between the supplysystem inductance and the line capacitance.

The chief specific object of the invention is to extend this principleto the point at which the reactance of the inductive element actuallydecreases with frequency above, say, 500 cycles at about the same rateas the reactance of a capacitor decreases with frequency. By properselection of the constants of the capacitor and of the inductiveelement, the two reactances may be made substantially equal and socancel each other over a wide range of frequencies in the usual voicerange. The shunt over such range acts substantially like a pureresistance.

Another specific object is to provide a sub stantially non-resonantshunt which at the fundamental frequency, usually cycles, acts as if itwere substantially a pure capacitance, while from, say, 300 cyclesupwards, it acts as if it were substantially pure resistance, the valueof which canibe predetermined. Such a shunt has a number of importantapplications.

It can be used to terminate'a line with a shunt which at frequencieswell above the fundamental frequency acts like a relatively lowresistance and thus prevents the building up of harmonic currents due toreflection. Reflection is strongest when the load on the line is weakestand is substantially absent when the balanced and residual circuit areterminated by a resistance equal to the respective characteristicbalanced and residual impedances of the line. The characteristicimpedance of a power line is represented approximately by /L/C, L and Cbeing the inductance and capacitance constants of the line per unit oflength. For an average overhead single-phase line this is in the orderof 500 ohms. For an average three-phase line it is in the order of 800ohms.

Such a shunt acts as a wattless load at the fundamental frequency, butas a resistance load,

at the harmonic frequencies. It is not always necessary to place it atthe end of or adjacent the end of the line. It may often be used toadvantage near the middle of the line, especially when the telephoneexposure is near the end of the power line.

The above form of shunt is also useful as a means for reducing the flowof harmonics from the supply source, whether they originate in thegenerator or supply transformer. In fact, the best arrangement is whereone non-resonant shunt is placed across the line adjacent the supplytransformer and the other is used to terminate the line.

The preferred form of shunt forming the sub- Ject-matter of the presentinvention consists of two parts in series with each other. The first isa capacitor which offers very considerable impedance to (SO-cyclecurrent. The second consists of a reactor and a resistor in parallel,the constants of which are chosen so that at 60-cycle current theimpedance of the reactor is considerably less than that of the resistor.As a result the greater part of the 60-cycle current which the capacitorpermits to flow goes through the reactor. As the current through thecapacitor and reactor is wattless, it follows that little of the60-cycle current passing through the shunt is expended in uselessgeneration of heat.

Various suitable applications of the invention are illustrateddiagrammatically, by way of example, in the accompanying drawings,wherein:

Fig. 1 illustrates a single-phase power system paralleling a telephoneline and embodying the present invention;

Fig. 2 illustrates a single-phase power system embodying a non-resonantshunt constructed in accordance with the present invention;

Fig. 3 illustrates a three-phase power system with shunts similar tothose shown in Fig. 2;

Fig. 4 is a graph showing various reactancefrequency relationships; and

Fig. 5 is a graph showing further reactancefrequency relationships.

As shown in Fig. 1, I0 represents a power line supplying energy to asecond multigrounded single-phase power line H by a transformer l2. Atelephone line running parallel to and in close proximity to the powerline Ii is indicated by [3. Across the line i l, preferably before itcomes near the telephone line, is connected a shunt designated generallyas M for reducing the higher harmonic currents in the line H beyond itspoint of connection. If a resonant condition exists between the supplysystem inductance and the line capacitance, harmonics in the vicinity ofthe resonance point and originating either in the supply system, loadtransformers, or loads will be increased by such resonance. The shunt Hadjacent the supply system will destroy this resonance condition,thereby reducing the harmonies, from both the supply system and loads.

However, when the line H is near or above one-quarter wave length atimportant frequencies, it may be necessary to place a second shunttoward the far end of the line to prevent the build-up of harmonics fromresonance in the line itself. This second shunt would be especiallyimportant if a harmonic generating load such as a. rectifier R were outon the line. The second shunt i4, placed adjacent the rectifier, willact as an additional preventative to the flow of harmonies back towardthe first shunt as well as outwards toward the open end of the line.

Each shunt comprises a capacitor l5 and a reactor IS in seriestherewith. The reactor I8 is provided with an iron core constructed sothat it has relatively high eddy current losses at frequencies in theusual voice range. With an iron core having thick laminations,the'reactance of the coil does not increase nearly as rapidly withfrequency as would be the case if such iron core or its equivalent werenot used.

As a capacitor acts as a short-circuit for lightning discharges, whilean inductance blocks them due to the very high frequency (steep wavefront) of such discharges, a gap shunt I1 is, to advantage, connectedaround the reactor to by-pass such discharges. Since the 60-cyclevoltage across the reactor is relatively small, there is no tendency forpower follow-up after the gap breaks down from surges.

The power system and shunt arrangement of Fig. 2 is broadly similar toFig. 1, but the shunts I 8, l9, and 20 are markedly diiferent in that aresistor 23 is connected in parallel with the reactor 22 to enable theinductive reactance of the combination of resistor and reactor todecrease with frequency above, say, 500 cycles at about the same rate asthe capacitance reactance of the capacitor 2|.

The shunts may have a condenser in series with the resistor, such as thecondenser 45 of shunt l9, to reduce the power loss at the fundamentalfrequency and lower the frequency at which the shunt as a whole actssubstantially as if it consisted solely of the resistor.

To avoid making the insulation on the reactor unduly heavy, gap shunts24 may be provided, as shown, to short-circuit lightning discharges.

While air-core inductance coils may be used when they are connected inparallel with a resistor, iron-core coils are ordinarily used as beingcheaper. Since the circuit is essentially 2. high loss circuit atharmonic frequencies, the socalled figure of merit Q of the reactor canb small in comparison with the Q of reactors de signed for conventionalresonant shunts. Iron cores tend to generate harmonic currents, thlgreater the flux density the greater the volume 0:. harmonics. Hencewhere reactors have been used previously, as in shunts tuned to oneparticular frequency, it has been customary to use air cores instead ofiron cores. I have found that when a reactor and a resistor are used inparallel, the resistor very greatly reduces the volume of harmonics, sothat the flux density may range between 30,000 and 50,000 or even 60,000lines per square inch without causing trouble.

As shown, there are three such shunts, one I8 near the supplytransformer to prevent harmonics generated in the supply system reachingthat part of the line which parallels the telephone line. a second i 9to prevent harmonics generated by a particular load, such as therectifier R, and a third 20 acting as a line termination shunt toprevent the building up of harmonic currents due to reflection. Ingeneral, the use of any one of the shunts II, It, and 20 reduces theground return harmonics in all parts of the line, so that all three areneeded only in exceptionally bad situations. It is especially noteworthythat the shunt II will very greatly reduce the fiow of harmonies fromthe rectifier R to the end of the line and that even if the shunt I isnot used, the line terminating shunt' 20 will largely prevent therectifier R sending harmonics along the line toward the supplytransformer if by destroyin resonance.

Attention is also called to the fact that when a shunt is used at-thesupply end only on a very long line the harmonics originating in theloads may still give rise to important resonance effects. This resonanceis prevented by the use of a terminating shunt toward the end of theline, although not necessarily at the precise end. Good results areoften obtained where the shunt is half-way or two-thirds the way to theend.

Rural power systems frequently consist of a network of lines withnumerous branches, and the impedance curve of the system is oftencomplex due to secondary reflections from the branch lines. To preventthese reflections, the branch lines which are electrically long atharmonic frequencies may be terminated in addition to the main linetermination. It is also possible to break up these reflection efifectsby a number of terminating networks distributed along the line, theindividual networks having relatively high impedance (such as 1,000 or2,000 ohms) compared with the characteristic impedance of the line. Inthe case of an electrically long singlephase branch from a three-phasefour-wire system, a simple and inexpensive treatment would consist of afew low-voltage non-resonant shunts on the 110- or 220-volt windings ofload transformers along the line at a spacing of, say, one 1 kv.-a shuntper mile. Fig. 3, shunt l3, illustrates this type of application.

In the case of a single-phase branch consisting of two phase wires froma three-phase system, a termination connected between the phase wireswill not prevent resonance in the residual circuit. In a single-phaseline of this type, a termination should be connected between each wireand ground. An alternative arrangement would consist of a three-phasetermination or nonresonant shunt to ground on the three-phase line at ornear the point where the single-phase branch connects to the three-phaseline. This latter arrangement may be used to reduce the earth currentsdue to the unbalancing efiect of a single-phase branch from three-phaselines of any type, i. e.. delta, unigrounded, or multigrounded systems.In the case of the unigrounded system, the preferred arrangement wouldinclude a termination connected between the neutral wire and ground.

In the average three-phase rural power system the impedance of the threephases will differ due chiefly to differences in the lengths andlocations of single-phase branches. Consequently, the resonance betweenthe line capacitance and the supply system inductance will occur at adifferent frequency for each of the three phases. This results inrelatively large residual currents in the three-phase section at thenon-triple harmonic frequencies (5, '1, 11, etc. harmonics), which in abalanced system would have 120 phase relationship between phases andtherefore cancel in the residual circuit. A non-resonant shunt connectedto ground as in Fig. 3 and located either adjacent a high impedancesupply system or at a considerable distance from the supply transformer,when the latter is very large, will be particularly effective inreducing the residual currents in both directions from the shunt, due todestruction of resonance in the residual circuit and to a balancingaction tending to bring the non-triple components into phaserelationship. Similarly, a terminating network toward the end of a long.line will tend to equalize the impedance of the three phases and beparticularly effective in reducing residual components. This action isimportant because it is usually the residual current (or morespecifically the current which appears as an earthreturn current) thatcontrols the overall influence of a power line on parallelingcommunication circuits.

The resonant frequency of the residual circuit of a power line willusually differ from the resonant frequency of the balanced circuit dueto such factors as loads and Y-delta transformers with grounded neutral.The non-resonant shunts described in this application due to theirnon-selective characteristics are effective irrespective of the resonantfrequencies.

Situations are sometimes encountered in which a pronounced condition ofresonance exists in a long, high-voltage transmission line such as60,000 or 110,000 volts. A terminating network placed directly on a lineof such high voltage is expensive due to the high voltage ratingrequired for the capacitors. Such lines generaily supply large loadcenters through stepdown transformers and in such cases the resonance inthe high voltage line may be destroyed by connecting a non-resonantshunt across the low-voltage windings of one or more load transformersor the line adjacent thereto. This permits a cheaper installation sincelower-voltage capacitors may be used.

Fig. 3 shows a series of Y-connected non-resonant grounded shunts of thetype described in connection with Fig. 2, applied to a three-phasefour-wire power line. Y-connected grounded shunts are usually preferableto either ungrounded Y-connected or delta-connected shunts because theformer are capable of destroying resonance in the residual as well asthe balanced circuits, while the latter do not destroy resonance in theresidual circuits. A paralleling communication line is indicateddiagrammatically at 32. In this case the three-phase conductors 26 arefed from a suitable delta-star transformer 25. A non-resonant shuntnetwork 20 is connected between the conductors 26 and the neutral 2!adjacent the supply transformer.

This shunt network comprises three shunt units, each consisting of acapacitor, reactor and resistor, as in the case of the shunts i8, i9,and 20 of Fig. 2. One end of each shunt unit is connected to a common orneutral point 20 connected by a conductor 30 to the neutral 3i.

Toward the open end of the line is a terminating shunt network 33comprising three shunt units 34, each consisting of a capacitor, reactorand resistor. These units are connected together at I and this common orneutral terminal is connected through a unit 35, similar to 34 exceptthat no capacitor is used, to the neutral 3i, where, as shown, theneutral is multigrounded, to provide a ground connection.

' The effect of the unit 35 is to shift the minimum point of theresidual impedance of the network to a lower frequency than that of theunits 34. For example, if the units 34 are designed to have a minimumpoint at 300 cycles, the constants of the unit 35 may be chosen to givea minimum point at 180 cycles in the residual circuit. This would beadvantageous, for example, where the magnitude of the 180-cyclecomponent from transformer exciting currents was sufficiently great torequire correction. In general, however, where the shunts 34 aredesigned for a minimum point as low as 300 cycles, the neutral element35 is not required. The impedance of the shunt to residual components insuch case will be one-third the impedance of unit 34.

If it is desired to terminate both the balanced and residual circuits insubstantially a pure resistance at harmonic frequencies and equal to thecharacteristic impedance of the line, the unit 35 should include acapacitor, and the constants chosen such that units 34 match thecharacteristic impedance of the balanced circuit of the line (usuallyaround 350 ohms) and unit 35 in series with the parallel impedance ofunits 34 matches the characteristic impedance of the residual circuit ofthe line (usually around 250 ohms). This four-capacitor arrangement is,however, more expensive and a sufficiently close impedance match isobtained in practice with capacitors in units 34 only.

Connected to the power line is a single-phase extension 40 supplyingpower to the primary of a transformer 4|. The secondary of suchtransformer supplies power to a load L through line 42. Across the line42 is connected a non-resonant shunt 43 of the type above described. Theshunt 43 prevents the extension line from acting as a capacitance whenthe load L is zero or very small and hence greatly reduces reflections,resonance and other injurious effects. The line 42 although short may,especially in conjunction with its feeder line 40, be regarded as apower line.

The operation of the non-resonant shunts of Fig. 3 will be explained byassuming the following values for the constants of the shunt:

Capacitor 1.76 mfd. capacitance or C.

Reactor 185 milihenries inductance or L.

Resistor 370, 500, 740, 1300 and ohms resistance or R.

These constants are suitable for shunts for a three-phase 12 kv. powerline using 6900-volt capacitors (voltage from line wires to neutral).

Curve A of Fig. 4 represents the variation in reactance with frequencyof the above-mentioned capacitor. Curve B shows the same thing for thereactor, when R is Curve C shows the variation with frequency of thereactive component of the total impedance of the reactor and resistorcombination when R is 1300 ohms. Curves D, E, and F show thecorresponding relationships when R is 740, 500 and 370 ohms,respectively. It will be noted that, when R is 3'70 ohms, from 500 to2,000 cycles and above, curves A and F almost coincide, i. e., thecapacitive and inductive reactances of the shunt are substantially equaland, being 180 out of phase, neutralize each other.

The vector sum of the reactances indicated by the curves A and F isplotted in Fig. 5 as curve H. Curve G is the resistance component of thetotal impedance of the reactor and resistor cdmbination when R is 370ohms. The vector sum of the resistance. curve G and the reactance curveH is plotted as curve J. It will be noted that the over-all impedance ofthe shunt has a minimum point of 210 ohms at around 300 cycles fromwhich point it increases asymptotically towards 370 ohms.

While such a shunt as that just described has a minimum impedance point,the differences between the impedance at that point and at higherfrequencies is so small compared with those obtained with straightcapacitance and inductance shunts that this shunt may be termed anonresonant shunt. To show how markedly differently my shunt operatesfrom straight capacitance and inductance shunts, curve K has been addedwhich represents the over-all reactance of a shunt having the same C andL, but having 13: i. e., when the shunt has only capacitance andinductance;

If it is desired to retain the minimum point at around 300 cycles, asshown in Fig. 5, but change R to some other value than 370, therequisite corresponding alterations in C and L can be computed from thefollowing formulas:

R=2L L=325/C where R=resistance of the resistance element in ohmsL=inductance of the reactor in milihenries C=capitance of the capacitorin mfds.

and the resistance The following table gives the values of R and L for Cfixed at 0.88 mfd.

Frequency of R L 0 minimum point It will be noted that the ratio of R toL changes from around 1:1 to 10:1 as the fre quency of the minimum pointgoes up from 150 to 1500 cycles. A ratio of 1:1 in the balanced circuitresults in substantial losses at 60 cycles and usually the ratio isaround 2: 1 or more. For the residual circuit lower ratios may be usedas there is substantially no 60-cyc1e current to be contended with. I

To change the relative values of C, L, and R for any desired minimumpoint, the first set of formulas is used. It will be noted, by applyingthe formulas to the figures given in the table, that for any fixedvalues of R and L, the value of C may be decreased by raising thefrequency of the minimum point. Hence, from the point of view of costalone, the higher the minimum point the better. From the purelyoperative point of view, the reverse is usually true.

If a line-terminating shunt has a minimum point at a relatively highfrequency, say 1,500, then resonance between the shunt on the one handand the supply system and line between such system and the shunt on theother hand may occur at some frequency below 1,500 cycles. If such shuntwere tuned with the supply system and intervening line to resonate at250 cycles, then the 180- and 300-cycle current from the supply systemwould travel the entire length of the line. Also any 180- or 300-cyclecurrents generated by the loads on the line would be increased. Usually,however, the 180- and 300-cycle current is of negligible importance.Where the 180- and 300-cycle currents are of importance, R should equal3L or less.

Even with the shunt near the supply system, it is advantageous to useone with a relatively low minimum point when there is a large amount oflow harmonics as it reduces the flow of both low and high frequencies.Thus, with an abnormal amount of 300-cycle current, a shunt with aminimum point around 300 is advantageous although the cost is greaterthan for a shunt with a minimum of 600. If the low harmonics are notimportant, then a shunt with a high minimum point tuned with the supplysystem can be used, because, while the low harmonics may be increased,that increase often is of small consequence and the installation cost isreduced by cutting down the size of capacitor required. Where a shunthaving a high minimum point, say 1,000 to 1,500 cycles, is locatedadjacent the supply system, the constants of the shunt should be suchthat the shunt resonates with the supply system at below 500 cycles,preferably at about 250.

In some cases, particularly where the line itself has considerabledistributed capacity, it is desirable that the inductive reactancecomponent of the line-terminating shunt be in excess of its capacitivereactance component. Such a condition would result from the constantsrepresented by curves A and E, or in some instances A and D,

.of Fig. 4. In that case the distributed capacity of the line largely,if not wholly, cancels out the excess inductive reactance of the shunt.Such shunt is especially adapted for terminating a power line. l

Where a terminating network is designed to match the characteristicimpedance of the line and is located at the end of the line, the lineimpedance, as seen from the sending end, is practically a pureresistance. Where it is desired to terminate a line in itscharacteristic impedance, R. should be around 500 ohms for single-phaseand 300 ohms for three-phase lines. The chief advantage of such a shuntis that it can be applied to any power line irrespective of its length.

That does not mean, however, that in any particular case where thereduction of harmonics is the sole desideratum R. may not vary widelyfrom the figures above given. One function of the termination shunt isto introduce losses in the line at harmonic frequencies and thereby dampout resonance effects. This last statement applies also to shuntsadjacent the supply system or between the ends of the power line.

A three-phase line termination shunt may be connected so as to terminatethe line only as respects the residual circuit. In that case the phasereactors are omitted and the three capacitors are each connected at oneend to each other and at the other end tc one of the phase wires. Thecommon or neutral terminal of the capacitors is connected through areactor and resistor in parallel to ground.

This arrangement may be used where the resonance in the residual circuitonly of the line is the important factor. This often occurs when theline is supplied by a direct-connected generator with its neutralgrounded. With direct-connected generators, the triple harmonicfrequencies are so outstanding that often they are all that have to beconsidered. To eliminate the effect of these triples, it is notessential to place a reactor-resistor combination in series with each ofthe three capacitors, one of such combinations in the line connectingthe three capacitors to ground being suihcient. Usually an over-allimpedance of 250 ohms is sufllcient, considering the three capacitors asbeing in parallel and in s..- ries with the reactor-resistorcombination.

I claim:

1. A power line system having a shunt across the power line for reducingcurrents therein of higher frequency than the fundamental, including acapacitor and in series therewith an inductive element having areactance which does not increase as rapidly as the frequency of thecurrents flowing therethrough in the usual voice frequency range.

2. A power line system having a shunt across the power line for reducingcurrents therein of higher frequency than the fundamental, including acapacitor and in series therewith an inductance coil having an iron corehaving high eddy current losses in the usual voice frequency range.

3. A power line system having a shunt across the power line for reducingcurrents therein of higher frequency than the fundamental, including acapacitor, a reactor in series therewith and a resistance, the reactorand resistance being connected in parallel with respect to each other.

4. In combination with a power line, a supply system therefor havinginductance, and a shunt connected across the line to reduce the currentstherein of higher frequency than the fundamental, such shunt including acapacitor and in series therewith an inductive element, the reactivecomponent of which increases markedly less rapidly than the frequency ofthe currents flowing therethrough in the usual voice frequency range,the capacitive reactance of the shunt being equal to the combinedinductances of the supply system and shunt at a frequency well below 500cycles.

5. In combination with a power line, a supply system therefor havinginductance, and a shunt connected across the line to reduce the currentstherein of higher frequency than the fundamental, such shunt including acapacitor and in series therewith an inductive element, including aninductance coil and a resistance, the coil and resistance beingconnected in parallel with respect to each other, the capacitivereactance of the shunt being equal to the combined inductances of thesupply system and shunt at a frequency well below 500 cycles.

6. A non-resonant shunt for reducing currents of higher frequency thanthe fundamental in power lines, comprising a capacitor and a reactorconnected in series with each other, and a resistor connected inparallel with the reactor, the impedance of the three parts of the shuntbeing so proportioned to each other that over a wide range 01frequencies the reactive component of the reactor and resistor combinedis of the same order of magnitude as the reactance of the capacitor.

7. In combination with a power line fed from a supply system by atransformer, a shunt connected across the power line adjacent thetransformer, such shunt comprising a capacitor and a reactor connectedin series with each other,

and a resistor connected in parallel with the reactor, the impedance ofthe three parts of the shunt being so proportioned to each other thatover a wide range of frequencies the reactive component of the reactorand resistor combined is of the same order of magnitude as the reactanceof the capacitor.

8. A non-resonant shunt for preventing the building up of high frequencycurrents due to reflections in power lines comprising a capacitor and areactor connected in series with each other, and a resistor connected inparallel with the reactor, the impedance of the shunt in the usual voicefrequency range being about equal to the characteristic impedance of thepower line.

9. In combination with a power line, a supply system therefor, a shuntconnected across the power line adjacent its end farthest from the saidsupply system, said shunt comprising a ca pacitor, a reactor connectedin series with the capacitor and a resistor connected in parallel withthe reactor, the total impedance of the shunt in the usual voicefrequency range being about equal to the characteristic impedance of thepower line.

10. In combination with a power line system, a load thereon of a typecreating harmonics and a communication line running parallel to thepower line, a capacitive shunt having high capacity relative toinductance connected across the power line between the point of supplyof power to the power line and a point not greatly beyond that at whichthe parallel relation between the two lines begins and a secondcapacitive shunt having high capacity relative to inductance connectedacross the power line between the harmonic-generating load and a pointnot beyond that at which such parallel relation ends, to reduce aplurality of the higher harmonics in that section of the power linesystem which runs parallel to the communication line.

11. In combination with a power line having considerable distributedcapacitance fed by a supply system having a relatively high inductanceand having, by virtue of such capacitance and inductance, a normalresonance at a frequency between 500 and 2,000 cycles, a shunt acrossthe power line including both a capacitor and an inductive elementhaving impedances such that the resonant frequency of the power line andsupply system is reduced to below 500 cycles, the inductance elementhaving a reactance which does not increase as rapidly as the frequencyof the currents flowing therethrough to reduce the flow of currents ofhigher frequency than 500 cycles in the power line.

12. In combination with a power line having considerable distributedcapacitance fed by a supply system having a relatively high inductanceand having, by virtue of such capacitance and inductance, a normalresonance at a frequency between 500 and 2,000 cycles, a shunt acrossthe power line, such shunt comprising a capacitor and a reactorconnected in series with each other, and a resistor connected inparallel with the reactor, the impedance of the three parts of the shuntbeing so proportioned to each other that at around 500 to 1,000 cyclesthe reactive component of the reactor and resistor combined is of thesame order of magnitude as the reactance of the capacitor, to destroyresonance above 500 cycles and to reduce the flow of currents of higherfrequency than 500 cycles in the power line.

13. A three-phase power line system having a Y-connected shunt acrossthe power line for reducing currents therein of 'higher frequency thanthe fundamental, including a capacitor in each branch of the shunt and aconnection between the neutral point of the shunt and the ground, saidconnection having in series therewith a reactor and resistor in parallelwith each other.

14. A three-phase power line system having a Y-connected shunt acrossthe power line for reducing currents therein of higher frequency thanthe fundamental, including a capacitor and in series therewith a reactorand resistor in parallel with each other in each branch of the shunt anda connection between the neutral point of the shunt and the ground, saidconnection having in series therewith a'reactor and resistor in parallelwith each other.

15. A three-phase power line system having a Y-connected shunt acrossthe power line for reducing currents therein of higher frequency thanthe fundamental, including a capacitor and in series therewith a reactorand resistor in parallel with each other in each branch of the shunt anda connection between the neutral point of the shunt and the ground.

16. A three-phase power line system havinga Y-connected shunt across thepower line for reducing currents therein of higher frequency than thefundamental, including a capacitor in each branch of the shunt and aconnection between the neutral point of the shunt and the ground, saidconnection having in series therewith a reactor and resistor in parallelwith each other.

17. A three-phase power line system having a Y-connected shunt acrossthe power line for reducing currents therein of higher frequency thanthe fundamental, including a capacitor and in series therewith a reactorand resistor in parallel with each other in each branch of the shunt anda connection between the neutral point of the shunt and the ground saidconnection having in series therewith a reactor and resistor in parallelwith each other, the resistance of each of the first three of saidresistors in ohms being greater than the inductance of the correspondingreactor in milihenries.

18. An inductive unit for reducing currents in power lines of afrequency higher than the fundamental frequency having an effectiveinductive reactance which decreases as the frequency of the currentflowing therethrough increases above a point in the lower part of thevoice range, comprising a reactor and a resistor connected in parallel,the resistance of the resistor in ohms being under 1500 and also between1.5L and 10L where L is the inductance of the reactor alone inmilihenries.

19. In combination with a three-phase power line, a Y-connected shunt,each arm of which includes a capacitor, a reactor connected in serieswith the capacitor and a resistor connected in parallel with thereactor, the impedance of the three elements of each arm of the shuntbeing so proportioned to each other that over a wide range offrequencies the reactive component of the reactor and resistor combinedis of the same order of magnitude as the reactan'ce oi the capacitor,and a connection from the neutral point of the Y-shunt to groundincluding a resistor and reactor in parallel with each other. theimpedance of the entire combination from the three lines to ground overa wide range oi. frequencies being substantially resistance, thecapacitive and inductive components substantially neutralizing eachother over such range.

20. In combination with a three-phase power 10 line, a Y-connected,capacitive shunt and a connection from the neutral point of the Y-shuntto ground including a resistor and a reactor in parallel with eachother, the impedance of the entire combination from the three lines toground over a wide range of frequencies being substantially resistance,the capacitive and inductive components substantially neutralizlngi eachother over such range.

HUGO W. WAHDQU'IST.

